The bonus damage component is in proportion to the %attack component of your skill. The bonus damage of a 150% skill is 1.5 times the bonus of a 100% skill. It’s the same for whatever class you use. Fireball of Wizard and Flame Slash of Ravager are both at 100%, so at lv13, they will both have 100% + 373 skill damage.

Now, the damage from sigils is additive to the bonus damage component of your skill.

For example: I’m lv43, Gunslinger as main (+3 skill levels) with a +1 Aimed Shot accessory so I have lv47 Aimed Shot. Aimed shot is 135%. At lv47, it will have 135% + 135%*8645 = 135% + 11671 attack. With a +919 damage sigil, and with my ~17k attack as shown on my damage stat, it will be like this: 1.35*17000 + 11671 + 919 = 35540 skill damage Aimed Shot.

So here are the damages of the individual class skills:

Grenadier

Fiery Salvo – 103%

Fiery Grenade – 95%

Plasma Cannon – 120%

Icy Grenade – 115%

Meteor Cannon – 125%

Wizard

Fireball – 100%

Icy Curse – 90%

Ice Storm – 85%

Storm Curse – 120%

Meteor Shot – 140%

Instant Thunder – 105%

Gunslinger

Aimed Shot – 135%

Bombardment – 155%

Sonic Bomb – 145%

Ultraviolet – 170%

Duelist

Slash Cut – 140%

Cross Slash – 130%

Sunflower – 132%

Whirling Dance – 135%

Deadly Wink – 190%

Balisong – 150%

Sorcerer

Devour Soul – 135%

Demon Seal – 95%

Binding Arrow – 120%

Blood Seal – 100%

Flame Impact – 135%

Guardian

Sword Slash – 140%

Shield Bash – 180%

Windfury – 115%

Thunder Cut – 130%

Terrifying Roar – 120%

Bard

Storm Song – 105%

Bliss Harmony – 125%

Heal Harmony – 30% (heal)

Ambient Drone – 140%

Healing Tones – 12% (heal)

Holy Notes – 13% (heal)

Ravager

Raging Slash – 105%

Horizon. Slash – 175%

Soaring Slash – 150%

Mirrored Axe – 95%

Flame Slash – 100%

Aftershock – 90%

Ultimates usually have 900% damage. The only one that does not use this, so far from the ones that I tested, is the Alessa + Bard ultimate, Linked Burst, which uses 600% with a Heal over Time side effect ~4x that of Ballad.